A communication system uses a differential edge modulation scheme having one of four or more values and a clock pulse is also encoded in each unit interval. A four-valued encoding may be as follows: prior to each unit interval, two signals are both at the same value; at the start of the unit interval, a first one of the signals is transitioned to indicate a first bit of the symbol; after a selected one of two delay periods that are less than the unit interval, the other signal to is transitioned indicate a second bit of the symbol; after this transition, both signals are again at the same value. The transitioning, at the start of the unit interval, provides an edge that may be extracted and used a timing reference.